Although optical wave plates have conventionally been used in optical pickup devices, liquid crystal displays, liquid crystal projectors and the like, it is necessary to have a function as a wave plate in a wavelength band of light used. For example, if it is a half-wave plate, it requires functions such as phase changes by 180° over a used wavelength band. In the case in which the half-wave plate is made of a single quartz crystal plate using birefringence of quartz crystal or the like, when an ordinary ray refractive index of the quartz crystal and an extraordinary ray refractive index thereof are set to be “no” and “ne”, respectively, and a thickness of the quartz crystal plate is set to be “t”, a phase difference Γ between the ordinary ray and the extraordinary ray when a light having a wavelength λ transmits through the half-wave plate is shown as Γ=2π/λ×(ne−no)×t, where the phase difference depends on the wavelength λ.
A broadband wave plate whose phase difference is approximately constant in a desired wavelength band has been disclosed in Patent Literature 1. A quarter-wave plate 40 shown in FIG. 12(a) is composed of a half-wave plate 41, an adhesive agent 42 and a quarter-wave plate 43. As shown in FIG. 12(b), with respect to a polarizing direction of a linearly polarized light incident on the quarter-wave plate 40, a streching axis of the half-wave plate 41 is positioned in a direction of −15° and a streching axis of the quarter-wave plate 43 is positioned in a direction of −75°. Here, the angles of the streching axes are described as angles at which a direction right from a y-axis is a positive direction within a yz plane. The half-wave plate 41 and the quarter-wave plate 43 are obtained by stretching and processing a polymer film of polycarbonate material. It is disclosed that the quarter-wave plate 40 functions as an approximately complete quarter-wave plate that is not dependent on wavelengths in a visible light range (400 nm-700 nm), where the function of the quarter-wave plate 40 is explained using a Poincare sphere.
Additionally, a laminated wave plate having a function as a half-wave plate by laminating a plurality of crystal plates has been disclosed in Patent Literature 2. FIG. 13(a) is a perspective view showing a structure of a half-wave plate 44, which is formed by laminating together quartz crystal plates 45 and 46. FIG. 13(b) is an exploded perspective view of the half-wave plate 44. The structure is disclosed in which the quartz crystal plate 45 having a phase difference Γ1 of 190° and an optical axis azimuth angle θ1 of 19° with respect to a wavelength of 420 nm are bonded to the quartz crystal plate 46 having a phase difference Γ2 of 200° and an optical axis azimuth angle θ2 of 64°, similarly, with respect to the wavelength of 420 nm in such a manner that their respective optical axes 49 and 50 intersect at an angle of 45° so as to function as a half-wave plate as a whole in high bands of wavelengths from 400 to 700 nm. As shown in FIG. 13(a), it is disclosed that it has a function in which when a P polarized light 47 is incident on the half-wave plate 44, its phase is deviated by 180° on a light exiting surface, so that a polarizing plane of the incident light is rotated by 90° to be converted into an S polarized light. In addition, it is disclosed that a relationship between the optical axis azimuth angles θ1 and θ2 is expressed by θ2=θ1+45° and 0°<θ1<45°.
The function of the half-wave plate 44 is explained using the Poincare sphere. In a detailed analysis, when respective Muller matrixes of the crystal plates 45 and 46 are set to be A1 and A2 and respective Stokes vectors indicating incident and exiting polarization states are set to be T and S, the Stokes vector S is expressed by the following formula:S=A2·A1·T  (1)
The phase difference of the half-wave plate 44 can be obtained from a component of the Stokes vector S.    [Patent Literature 1] Japanese Unexamined Patent Publication No. 10-68816.    [Patent Literature 2] Japanese Unexamined Patent Publication No. 2004-170853.